A sinker electric discharge machining apparatus intermittently applies a voltage to a microscopic work gap that is filled with dielectric fluid. If the insulating properties of the dielectric fluid break down, electric discharge occurs in the work gap, and an ON time commences. During a controlled ON time, a current pulse is supplied to the work gap, and the workpiece material is removed. In order to restore the insulating properties of the dielectric fluid, application of voltage is stopped during a controlled OFF time.
If the insulating properties of the dielectric fluid are not sufficiently restored, undesirable sustained arc discharge occurs during the ON time. With sustained arc discharge, a current of high current density flows through the work gap. However, sustained arc discharge does not contribute to the removal of workpiece material. During sustained arc discharge, it is known that the voltage of the work gap (hereafter referred to as gap voltage) is low compared to transient arc discharge, which is normal electric discharge.
If the insulation characteristics during the OFF time deteriorate further, an abnormal state that has significant adverse effects on the machining of the workpiece arises in the work gap. An abnormal state is, for example, continuation of sustained arc discharge or concentrated electric discharge. Continuation of sustained arc discharge inflicts excessive wear of the tool electrode or irreversible damage on the workpiece. Concentrated electric discharge where electric discharge only occurs at the same location on the workpiece also inflicts damage on the workpiece. According to one approach, this type of abnormal state is avoided by significantly increasing the OFF time. However, an excessively increased OFF time adversely lowers machining efficiency.
Patent publication 1 discloses an electric discharge machining method that first rapidly extends an OFF time from an appropriate initial value to a suitably large specified value, when abnormal electric discharge has occurred. After being rapidly extended, the OFF time is then reduced in a stepwise manner in short units of time. The disclosed electric discharge machining method can avoid abnormal states and shorten machining time.
Patent publication 2 discloses an electric discharge machining method in which when an abnormal state is detected, an OFF time is extended from a reference value, and when a normal state is detected, the OFF time is reduced from the reference value. With this electric discharge machining method, steps for extending or reducing the OFF time are counted, and the reference value for the OFF time is changed in accordance with this step count. As a result, the OFF time control system is stable.    Patent Publication 1: Japanese patent No. 57-36096 Patent Publication 2: Japanese patent No. 2564389